The present invention relates to drilling fluid additives used in oil and gas drilling, and more particularly to wood-based additives that do not employ separate binding agents.
Subterranean wells are commonly dug for the purpose of reaching desirable oil and gas deposits beneath the earth. It is the usual practice when drilling wells into the earth by means of rotary drill machinery to introduce a drilling fluid into the well as the well is being formed. These fluids (often referred to as “mud”) are used to sweep away cuttings from the cutter head as a well is drilled. The fluids also serve the purpose of cooling the cutter head during the drilling head operation, since the cutter head is subject to intense heat as a result of friction and subterranean pressures during the cutting process. After being introduced downhole, the fluids are brought to the surface where they may be screened to remove cuttings and other particulate matter, then reused. Many different formulations for drilling fluids have been employed in different environments, with both oil-based and water-based fluids being in widespread use today for various land-based and offshore drilling applications.
One of the problems with the use of drilling fluids is that the fluids may be lost during well boring. One reason for the loss of drilling fluids is that the fluid may seep into subterranean rock formations that have a high permeability to the material used for the drilling fluid. This is often exhibited as a slow but steady loss of drilling fluids being returned to the surface. Another reason for the loss of drilling fluids is that voids may be encountered in the subterranean rock formations, and drilling fluid may flow out of the well and into these voids. This problem may be exhibited as a sudden, dramatic drop in the return of drilling fluids to the surface. To avoid drilling fluid losses due to either or both of these phenomena, it is common to employ additives in the drilling fluid for the purpose of lost circulation control. The art includes numerous additives, often referred to as loss circulation materials or LCMs, that have been successfully employed in both water-based and oil-based drilling fluids. In a typical drilling operation, the loss circulation material is added to the drilling fluid in a “mud pit” adjacent to the drilling area. The drilling fluid is drawn from this pit in order to be used in connection with drilling, and is then returned to the pit after being drawn from the wellbore and screened to remove cuttings and other impurities. The use of a mud pit is also advantageous in that it allows the drilling fluid to cool before being returned to the wellbore for repeated use.
A number of attempts have been made to use wood-based materials as a loss control material in drilling fluids. For example, U.S. Pat. No. 5,763,367 teaches the use of a drilling fluid additive that may include ground wood fiber. U.S. Pat. No. 6,630,429 teaches a lost circulation material that uses natural fiber material. U.S. Pat. No. 6,399,545 teaches a drilling fluid additive that may include yellow pine or yellow pine bark. Wood-based loss control materials are environmentally safe, and thus offer a distinct advantage in that regard over certain inorganic chemical formulations. Also, some wood-based loss circulation materials do not change the pH of the drilling fluid, which may also be important in certain drilling applications. Finally, wood-based materials are available at very low cost, providing another advantage to their use.
A significant problem with wood-based materials is the high density of the drilling fluid as compared to the lower density of commonly used wood-based materials. Because most wood-based additives are of a significantly lower density than the drilling fluid, they tend to float on top of the drilling fluid when added. The necessity of thoroughly mixing the wood-based materials into the drilling fluid slows down the well drilling operation, and has largely prevented widespread adoption of wood-based products as loss control materials. In addition, the light but bulky LCM material takes up significant amounts of storage space due to the low product density. While this is not typically a concern in land-based drilling operations, storage space is a very significant concern for offshore drilling, due to the limited space available on oil drilling platforms and the high cost of transporting bulky materials to the platforms by sea.
The prior art does include attempts to produce a wood-based loss circulation material that has a higher density than loose wood-based material. For example, U.S. Pat. No. 4,428,844 teaches a loss circulation material that is formed of ground paper pressed into pellets. The density of the pellets depends upon the pressure used during the pellet formation process. The examples stated in the patent include paper pellets with densities of about 31.5 pounds per cubic foot, 34 pounds per cubic foot, and 40.1 pounds per cubic foot. U.S. Pat. No. 6,825,152 teaches that the use of a pelletized, higher-density material speeds the incorporation of the material into drilling fluid. In this case, ground wood is taught as an example material. A cellulosic additive is taught with an uncompressed density of 10 pounds per cubic foot, with the pelletizing process resulting in a material with a density of about 17 pounds per cubic foot. This patent recognizes the need to provide additives in a form that mixes with the drilling fluid at a higher speed; nevertheless, the product is introduced into the drilling fluid in a pellet form, and must disperse into the constituent particles forming the pellets upon introduction to the drilling fluid in order to provide its loss control properties.
Despite these previous attempts to produce a workable, wood-based loss circulation material, wood-based loss circulation materials have failed to see widespread adoption. Investigations by the inventor hereof have led to the conclusion that there are two primary reasons for the failure of prior art wood-based LCM materials. First, the prior art has generally failed to recognize the importance of matching the density of the wood-based loss circulation material to the density of the drilling fluid, and thus the prior art does not include a wood-based loss circulation material that exhibits this property. Typical drilling fluids have a density of from about 67 to about 90 pounds per cubic foot. If the density of the loss circulation material is significantly lower than the density of the drilling fluid, then it will tend to simply float on top of the drilling fluid; likewise, if the density of the loss circulation material is significantly higher than the density of the drilling fluid, then the loss circulation material will simply fall to the bottom of the mud pit, and will not properly mix with the drilling fluid. The prior art does not include a wood-based loss circulation material with a density matched to the drilling fluid.
A second problem is that prior art wood-based LCMs have tended to change the electrical properties of the drilling fluid, whereas electrical stability is an important parameter in drilling fluid performance. The electrical stability of an oil-based drilling fluid relates to its emulsion stability and oil wetting capability, and is generally tested by measuring the current between two charged plates across a varying voltage. The test results generally relate to a drilling fluid's oil-wetting of solids and to stability of emulsion droplets, although the interrelationship of these factors is not well understood. The introduction of any suspended solids tends to change the electrical properties exhibited by the drilling fluid. The inventor has thus recognized that a wood-based LCM material that does not change the electrical properties of the drilling fluid would be highly desirable.
The present invention overcomes the limitations of the prior art, solves the problems identified by the inventor with respect to the prior art, and presents certain advantages over the prior art as set forth below.
References mentioned in this background section are not admitted to be prior art with respect to the present invention.